Retarding skirt weld cracking in coking vessels



Jan. 26, 1965 D. J. BERGMAN 3,167,486

RETARDING SKIRT WELD CRACKING IN COKING VESSELS Filed Dec. 15. 1961 Figure l //V l/E/V 7'0 R: Donald J. Bergman A TTOR/VEYS 3,167,485 RETARDHLJG SKERT WELD CRACKING IN ZQEGNG VESSELS Donald J. Bergman, Kenilworth, EllL, assignor to Universal Gil Products (Iompany, Des Plaines, i l-1., a corporation of Delaware Filed Dec. 15, ran, Ser. No. 159,713 It? Claims. (ill. 196-133) This invention relates to an improved pressure vessel design and more particularly to means for preventing or retarding skirt weld cracking of coke chambers utilized in the familiar delayed coking process.

The operation of a typical delayed coking process is well known and requires little elaboration here. A heavy liquid hydrocarbon charge is heated to coking temperature, i.e. about 800950 F., and is then passed in a partially vaporized state into the top or bottom of a coke chamber which is usually a vertically elongated vessel; two or more coke chambers are manifolded in parallel so that one may be decoked While another is on stream. Under the coking conditions prevailing in the coke chamber, a portion of the charge is converted to coke via a complex series of reactions involving cracking and polymerization, which coke is deposited on the bottom and the walls of the coke chamber; light ends and other unconverted material are Withdrawn from the chamber through a vapor transfer line for further treatment and/ or recycle. The coke accumulation progresses upward and when it reaches about 75 %80% of the vessel volume after about 8-30 hours of operation, the fiow from the coking heater is switched to an empty coke chamber, and the connecting lines are blanked oil for cleanout'. The coke chamber pressure is reduced to a sump, allowing vaporization of liquid from the top of the coke bed, and steam is introduced at the bottom of the vessel in order to further vaporize liquid and to initiate cooling. After steaming for a period of about 1 hour, water is pumped into the bottom of the chamber to complete the cooling or quenching step. Following quenching the coke chamber is unheaded and the coke is removed. In the early days of the art, the coke was removed by a cable or by mechanical drilling but it is almost universal practice today to remove the coke by means of hydraulic jets obtained from water under high pressure; the jets cut through the body of coke to the metal wall of the vessel, and generally strip the wall free of coke. Each coke chamber therefore undergoes a periodic accumulation and removal of coke on a 24-48 hour cycle.

Such operation subjects a coking vessel to unusually severe temperature shocks as it is cycled through a temperature range of about ambient to about 850 F. every 24 hours or so. The startup operation and the quenching step produce especially severe local temperature gradients in the vessel walls. During startup, following steaming to remove air from the system, hot vapor is bypassed back to the empty chamber to start heating it up; this is continued until the temperature of the vessel is raised to 600-700 F. Then the charge stock is cut in, causing a sudden temperature rise to about 850 F. During the cooling step when water is admitted into the bottom of the chamber, this is converted into steam by the hot coke which often results in high local pressures because of the large volume of steam generated, and promotes a rapid drop in temperature of the vessel walls. This temperature drop proceeds up the chamber as the coke becomes cooled down to water temperature. Hence it is possible for the bottom of the vessel to be at a temperature of 100 F. while the top thereof is still at a temperature of about 700 F.

Coke chambers for commercial delayed coking process units generally range in diameter from 10 to 20 feet, in length from 30 to feet between tangents, and in shell thickness from /8" to 1%". A coke chamber is normally a vertical vessel having a cylindrical body with conical or elliptical heads supported at the lower portion thereof by a cylindrical or conical skirt member. A skirt support has een recognized as the most dependable type of support for a vertical vessel because weight and wind loads are more or less uniformly distributed around the circumference of the vessel and local stress effects are minimized; in addition, a skirt type support provides a temperature transition zone from the cold vessel footing to the hot vessel shell without need for expansion provisions. The skirt support is usually fabricated of to 1" plate and extends 3 to 5 feet in length depending upon vessel weight and diameter and upon its design operating temperature. The skirt is connected to the vessel by means of a circumferential Weld, and may be lap-welded to the cylindrical wall of the vessel or may be butt-welded to the curved or knuckle portion of the lower conical head. This skirt attachment weld is particularly sensitive to the severe temperature stresses produced in a coking vessel by reason of the aforesaid operating conditions inherent in its use and by reason of the distortion caused by temperature gradients. Skirt weld cracking is considered to be one of the most common and frequent types of structural failure associated with coking vessels. It is caused primarily by high heat flow axially through the skirt during the coking operation and by rapid temperature changes during the vessel warmup and quenching periods which may induce transient stresses much in excess of steady-state stresses. This problem has led to the development of the so-called temperature transition skirt wherein the skirt is lengthened and made thinner and is completely insulated, or insulated over the upper or A portion thereof. Various other techniques have been proposed to reduce the tendency toward skirt weld cracking such as increasing the vertical Weld leg and increasing the flexibility of the skirt by slotting its upper portion. However, none of these methods has met with any pronounced degree of success in reducing skirt weld cracking.

It is an object of the present invention to reduce temperature gradients in and about the skirt weld by creating a coke depositing and retention zone in a limited region within the vessel near the skirt weld and utilizing the deposited coke itself as an internal layer of insulation.

A broad embodiment of this invention relates to a means for reducing thermal deformation of the welded juncture between a coking vessel wall and a vessel-supporting member which comprises an internally disposed baffle member proximately spaced from the interior surface of said wall adjacent to the internal projection of said welded juncture, said bathe member being substantially coextensive with the length of the welded juncture and extending beyond the internal projection of each longitudinal edge thereof a distance, measured from the longitudinal centerline of the juncture, of from about 3 to about 30 times the thickness of said Wall at the juncture.

A more specific embodiment of this invention is directed to an improvement in the construction of a coking vessel having a vertical cylindrical wall and a conical head section connected to the lower end of said wall, said coking vessel being supported by a skirt member Welded to the head section around the circumference thereof near the junction of the cylindrical Wall with the head section, which improvement is a means for reducing thermal deformation of the welded juncture between said head section and said skirt member which comprises an openended frusto-conical baffle member radially centered within said vessel and proximately spaced from the inenemas terior surface thereof adjacent to the internal projection of said welded juncture, said bafile member being substantially coextensive with the length of the welded juncture and extending above and below the juncture a distance, measured from the longitudinal centerline thereof, of from about 3 to about 30 times the thickness of said wall at the juncture.

Another specific embodiment of this invention is directed to an improvement in the construction of a coking vessel having a vertical cylindrical wall and supported by a skirt member Welded to said wall around the circumference thereof, which improvement is a means for reducing thermal deformation of the welded juncture b tween said skirt member and said wall which comprises an open-ended cylindrical bafile member radially centered within said vessel and proximately spaced from the interior surface thereof adjacent to the internal projection of said welded juncture, said baflle member being substantially coextensive with the length of the welded juncture and extending above and below the juncture a distance, measured from the longitudinal centerline thereof, of from about 3 to about 30 times the thickness of said wall at the juncture.

, The internal balile member in conjunction with the vessel wall forms a coke depositing and retention zone or pocket which is coextensive with the length or girth of the skirt weld and which extends beyond each longitudinal or circumferential edge thereof a distance of from about three to about thirty times the wall thickness, measured from the longitudinal centerline of the skirt weld or, translated in equivalent terms, the baffle has a width of 6 to 60 times that of the vessel wall thickness. As employed herein the term wall thickness refers to the thickness of the metal shell of the vessel exclusive of any insulation attached thereto; if the shell thickness of a particular vessel should be non-uniform in respect of vessel length, its transverse thickness at the longitudinal centerline of the skirt weld is intended as the reference dimension which establishes the width of the battle. If the vessel wall should be curved in the region of the skirt weld, its transverse thickness is to be measured along a line which is normal to the wall at said longitudinal center-line. The bai'lle is spaced a short distance inwardly from the interior surface of the vessel wall but is preferably connected thereto along one edge, the other edge of the baffle being spaced from the vessel wall to provide an opening or slot for admitting hydrocarbon feed undergoing coking into the coke retaining pocket. When the vessel is initially commissioned, its interior surface, including that portion bounded by the baffle member, is relatively clean and free of deposited coke. However, as the coking operation progresses a layer of coke will build up on the vessel wall. In particular, reactant fluid also enters the pocket between the bafile and the vessel wall and deposits coke upon the preferential coke-depositing surfaces of the pocket, and the latter soon becomes substantially filled with coke. Since the coke has a much lower thermal conductivity than the metal wall of the vessel, it acts as a layer or blanket of thermal insulation overlying the internal projection of the skirt weld. As the coking vessel becomes hot, the insulating coke layer reduces the outflow of heat transversely through the vessel wall and instead diverts and reduces the heat flow into the wall at points beyond the coke layer after which the direction of heat flow is radially outward and also longitudinally through the vessel wall. In other words, a substantially reduced portion of the heat flow which otherwise would seek the path of least resistance, which is transversely through the vessel wall into the skirt weld, is compelled to pass through a path length several times the wall thickness. The net effect is to mitigate thermal gradients in and about the skirt weld with corresponding retardation of skirt weld cracking by thermal deformation.

The baffle member further fulfills an important function in maintaining the insulating coke layer in place during the quenching and coke unloading steps. When cooling water is admitted into the vessel, its walls begin to contract. The coke, which has accumulated throughout most of the interior surface area of the vessel when the latter is hot, has a much lower temperature coefficient of expansion than the metal walls of the vessel; the rapid cooling of the vessel therefore sets up high internal pressures within the coke layer approximating its ultimate compression or crushing strength whereby the coke layer in various regions of the vessel tends to fracture. The baffle member, however, retains the coke layer in place and relatively intact even though it should suffer fractures or loosening by shock cooling. The baffle member is preferably made substantially imperforate-excluding the provision of weep holes for drainage of entrapped liquid and vent holes for release of air or gas-so that it elfeo tively shields the coke layer from the cutting action of the hydraulic jets employed in decoking. Although the hydraulic jets strip the vessel wall free of coke throughout most of the vessel volume, the limited region of the insulating coke pocket is protected therefrom. Thus, when the decokcd vessel is placed back on stream, the insulating coke layer is already formed in place and serves to guard the skirt weld from shock heating effects, as well as quenching effects, in this and subsequent cycles of operation.

The several embodiments of the invention and modifications incident thereto may best be described in conjunction with the accompanying drawings which are presented as illustrative of the best mode of practicing the in vention but not with the intention of unduly limiting its generally broad scope.

FIGURE 1 is a sectional elevation view of the lower portion of a coking vessel showing one arrangement of the coke retaining baffle member of this invention.

PEGURE 2 is a detailed elevation view of another embodiment of the bafiie member.

FIGURE 3 is a detailed elevation view of a third embodiment of the baffle member.

In FIGURE 1 there is shown in elevation only so much of the lower section of a vertically mounted coking vessel ll as is relevant to a discussion of the present invention. Details such as connecting lines, valve manifolds, and appurtenant vessel intervals have been omitted for the sake of simplicity. The lower section of vessel 11 comprises a vertical cylindrical wall 12 to which is connected, as by a girth weld, a lower conical head or cone 13. The upper portion of cone 13 comprises a curved or knuckle section 14 which brings the upper edge of the cone smoothly into alignment with the lower edge or tangent of wall 12; alternatively, the knuckle section 14 may be a separate course joining wall 12 with cone 13. The lower end of cone 13 is provided with a manhole neck which is normally sealed off by a removable bottom cover plate 15. Vessel ll is supported by a cylindrical skirt member 16 which is welded along its upper edge to knuckle section 14. be welded juncture therebetween is indicated generally at 17; the upper circumferential edge of the juncture is indicated at 24 and the lower circumferential edge thereof at 25. A layer of insulation 18 encases wall 12, cone l3 and the upper portion of skirt 16. The lower end of skirt l6 terminates in a T-flange 19 which bears upon and is bolted to a concrete support ring 26; ring 2% in turn is supported at the appropriate elevation above grade by means of columns 21.

The coke-retaining ballie member of the embodiment of FIGURE 1 is an open-ended, frusto-conical baffle 22 which is radially centered within vessel 11 and suitably connected along its upper edge, as by lap weld 23, to wall 12. The lower edge of baffle 22 is spaced inwardly a short distance from the interior surface of cone 13 to provide a narrow, relatively unobstructed circumferential opening 26 therebetween. It will be seen that battle 22 is disposed adjacent to the internal projection of skirt weld 17 and is coextensive with the length or girth thereof. Broadly speaking, bafi'le 22 should extend above and below the internal projections of edges 24 and 25 a distance, measured from the circumferential centerline of the skirt weld, of from about 3 to about 30 times the transverse thickness of the vessel wall at the weld. Thus, in the case of the vessel having a uniform wall thickness of 1", the upper edge of the battle 22 will extend from a minimum of 3" to a maximum of 30 above the circumferential centerline of juncture 17, and the lower edge of bafile 22 will likewise extend from a minimum of 3" to a maximum of 30" below said centerline.

In order to provide adequate skirt weld insulation while at the same time not unduly diminishing the useful coke-producing volume of the vessel, it is preferred that bafile 22 extend above and below skirt weld 17 a distance, measured from the circumferential centerline of the skirt weld, of from about 6 to about times the transverse thickness of the vessel wall at the weld or, translated into equivalent terms, the battle has a width of 12 to 30 times that of the vessel wall thickness. In such preferred case for a vessel having a uniform wall thickness of 1", the upper edge of bafile 22 will extend from a minimum of 6" to a maximum of 15" above the circumferential centerline of skirt weld 17, and the lower edge of baffle 22 will similarly extend from a minimum of 6 to a maximum of 15" below the circumferential centerline of skirt weld l7.

Baflie 22 is thus seen to provide an annular coke-retaining pocket 27 which is in open communication with the main interior of vessel 11 via circumferential opening 26. Opening 26 serves to admit coke-forming vapor or liquid into pocket 27; it may range in width from about 0.05" to about 1"; preferably such clearance should not exceed about 0.30", although this is not particularly critical. A few small, circumferentially spaced vent holes 28 are provided in baflle 22 near its upper edge to permit the escape of gas and/ or vapor from pocket 27. In the absence of such vent holes, inert atmospheric gases or light hydrocarbon vapors may become entrapped in the pocket and block the entry of coke-forming vapors thereto. If desired, vent holes 28 may be provided with jet shields in order to prevent intrusion of the hydraulic jets into pocket 27.

When pocket 27 becomes substantially filled with coke, there results a more or less permanent insulating layer of coke contiguous to and overlying welded juncture 17. The rate of transverse heat flow through the wall of knuckle section 14 is thereby greatly reduced, which reduces the temperature of juncture 17, thereby reducing the heat flow through the skirt and reducing the thermal deformation and stress at the jucture. Baffle 22 serves to retain the accumulated coke in the pocket even though it should become fractured by thermal shock and to shield such accumulated coke from the cutting action of the hydraulic jets during the decoking operation.

Proper design of the baffie member should include consideration of several potentially disruptive agencies. First, there is the possibility of bending or other structural damage to the baflle by falling coke from the upper portion of the vesselduring decoking; this is not expected to be a problem so long as sufiicient coke is present in pocket 27 as can act as a baffle reinforcing member to resist the impact loading produced by the falling coke.

Secondly, the accumulation of quench water in the pocket i should be avoided since the rapid reheating ofthe vessel after it is placed back on stream would cause'sudden vaporization of the entrapped water, and the attendant generation of explosive pressures-within the pocket may warp the baffle or fracture its connecting weld; however, since opening 26 is disposed at the lower end of pocket 27, it is free-draining so that whatever water should happen to get into the pocket during quenching can escape by gravity flow when the vessel is emptied. Thirdly, care should be exercised in the use of the hydraulic jets so that the jets are not deflected by the vessel wall into opening 26 because the pressure head developed through conversion of the very high kinetic energy of the jets within the confined volume of the pocket may be sulficiently high as to damage the baffle. In order to minimize the possibility of jet intrusion, the lower end of baffle 22 may be bent outwardly at an acute angle to wall 13 to provide a jet-deflecting baffle; alternatively, a separate jet-deflecting baffle may be attached to wall 13 a short distance below opening 26 and may be extended upwardly at an acute angle to wall 13 above opening 26.

FIGURE 2 illustrates a modification of the frustoconical baffie member. whereby it is connected to the vessel along its lower edge rather than its upper edge as in FIGURE 1. The portion of interest of the vessel wall includes a vertical cylindrical section 32, a knuckle section 34, and a cone section 33. A cylindrical skirt member 36 is welded along its upper edge to knuckle section 34. The welded junction therebetween is indicated generally at 37; the upper circumferential edge of the juncture is indicated at 44 and the lower circumferential edge thereof at 45. Ballle 42 is an open-ended, frusto-conical member which is radially centered within the vessel and is suitably connected along its lower edge, as by lap weld 43, to conical head section 33. The upper edge of battle 42 is spaced inwardly a short distance from the interior surface of cylindrical wall 32 to provide a narrow, relatively unobstructed circumferential opening 46 therebetween which communicates with pocket 4-7. As in FIG- URE 1, the baffle of this embodiment is disposed adjacent to the internal projection of skirt weld 37 and is coextensive with the girth thereof; the dimensional limitations of the bafile discussed in connection with FIGURE 1 also apply to the embodiment of FIGURE 2. This construction is perhaps less desirable than that of FIG- URE 1 since bafile 42 is more susceptible to damage by falling coke. A further disadvantage of this arrangement is that with the lower end of pocket 47 closed, it is not free-draining so that several small drain holes should be provided around the lower peripheral edge of baffle 42; alternatively, bafile 42 may be mounted so as to be slightly inclined to the horizontal and a single larger drain hole provided at its lowest point. If drain holes are employed they may be fitted with jet shields in order to prevent intrusion of the hydraulic jets into pocket 47.

FIGURE 3 illustrates another embodiment of the baffle iember which may be employed when the skirt member is lap welded to the cylindrical wall of the vessel rather than butt welded to its knuckle section or conical head section. The portion of interest of the vessel wall includes a vertical cylindrical wall 52 to which is welded cylindrical skirt member 56 along its upper edge. The welded juncture therebetween is indicated generally at 57; the upper circumferential edge of the juncture is indicated at 64 and the lower circumferential edge thereof at 65. Battle member 62 is an open-ended, cylindrical member which is radially centered within the vessel. The upper end of bafile 62 is bent outwardly to form an annular end closure or lip 69 which in turn is suitably connected to cylindrical wall 52, as by lap weld 63. Bafiie 62 in conjunction with wall 52 forms an annular coke-retaining pocket 67 which is in open communication with the main interior of the vessel via lower circumferential opening 66. As in the case of the preceding embodiments, baffle 62 is disposed adjacent to the internal projection of skirt weld 57 and is coextensive with the girth thereof. Battle 62 extends above and below the internal projections of edges 64 and 65 a distance, measured from the circumferential centerline of the skirt weld, of from about 3 to about 30 times the transverse thickness of the vessel wall at the weld, and preferably a distance of from about 6 to about 15 times such transverse thickness. Opening 26 may range in width from about 0.05" to about 1". It will be observed that pocket 67 is free-draining through opening 66 and that since the battle extends vertically and is relatively close to the wall of the vessel, it is less likely to be struck by falling coke than Where the bathe extends acutely toward the central portion of the vessel. A few small, circumferentially spaced vent holes may be provided in baflle 62 near its upper edge or in lip 69 to allow the release of gas and/ or vapor from pocket 67.

porting members other than skirt supports such as radial lugs or circumferentially' spaced columns whose welded juncture is elongated in a direction parallel to the central longitudinal axis of thervessel.

Various modifications of the invention other than those specifically treated therein are self suggestive and are included within the broad scope thereof. For example, the

coke-retaining baffle may be discontinuously connected to the vessel wall as spaced points along one edge instead of by a continuous weld. .The bafile may be held in position by means of internal bracework connected to the adjacent vessel Wall or to some other interior surface of the vessel, and both the upper and lower ends of the coke pocket may be left open. The batlle itself may obviously have a shape other than a surface of revolution, for example, it maybe constructed of a series of short, straight plates to form a polygonal structure.

I claim as by invention:

1. In combination with a pressure vessel utilized as a coking chamberand having an exteriorly extended vessel-supporting metal member welded in a relatively long narrow section to a relatively thin metal wall of said vessel, means for reducing thermal deformation of the welded juncture between said Wall and said supporting member which comprises an internally disposed baffle member connected to and proximately spaced from the interior surface of said'wall and opposite said welded juncture,.so as to form a coke depositing and retention zone, said bafiie member being substantially coextensive with the length of the welded juncture and extending be yond each longitudinal edge thereof and having a width of 6 to 60 times that of the metal wall thickness, and an insulating layer of coke in said coke retention zone.

2. ln combination with a vertical pressure vessel utilized as a coking chamber and having an exterior downwardly extending vessel-supporting metal member welded in a relatively long narrow section to a relatively thin metal wall of said vessel, means for reducing thermal deformation of the welded juncture between said wall and said supporting member which comprises an internally disposed bailie member connected -to and priximately spaced from the interior surface of said wall and opposite said welded juncture, so as to form a coke depositing and retention zone, said baffle member being substantially coextensive with the length of the welded juncture and extending beyond each longitudinaledge thereof and having a Width of 6 to 60 times that of the metalwall thick ness, and an insulating layer of coke in said coke retention zone.

. 3. In combination with a vertically elongated pressure vessel utilized as a coking chamber and having a vesselsupporting metal skirt member welded in a relatively long narrow section to a relatively thin metal nonhorizontal wall of said vessel around the periphery thereof, means'for reducing thermal deformation of the welded However, it is below the juncture and having a width of 6 to 60 times juncture between said wall and said supporting member which comprises an internally disposed battle member and opposite said welded juncture, so as to form a coke depositing and retention zone, and having the form of an open-ended surface of revolution one edge of whichis connected along at least a portion thereof to a nonho-rizontal wall of said vessel and the otherredge of which is spaced from the interior surface of said vessel, said baifiemember being substantially coextensive with the length of the welded juncture and extending above and below the juncture and having a width of 6 to 60 times I that of themetal wallthicknesa'and an insulating layer of coke in said coke retention zone.

4. The apparatus of claim 3 further characterized in thatbaftle member has a width of 12 to 30 times the thickness of said wall at the juncture. V 5. In combination with a cokingvessel having a relatively thin metal vertical cylindrical wall and a relatively thin conical metal head section connected to the lower end of said wall, said cokingIvessel being sup orted by.

a metal skirt member welded in a relatively long narrow section to said. head section around the circumference thereof near the junction of said cylindrical wall with the head section, means for'reducing thermal'deforrnation of the welded juncture between said head section and said skirt member which comprises an open-ended frustoconical baille member radially centered within said vescl andconnected to and proximately spacedfrorn the interior surface thereof and opposite said welded juncture, so as to form a coke depositing and retention zone,

said bafie member being substantially coextensive with the girth of the welded juncture and extendingabove and that of the metal wall thickness, and an insulating layer of coke in said coke retention Zone.

6. The apparatus of claim 5 further characterized in that said skirt member is cylindrical. j

7. The apparatus of claim 5 further characterized in that the upper edge of said battle member is connected'to said cylindrical wall and the lower edge thereof is spaced inwardly from said conical head section.

8. The apparatus of claim 5 further characterized in that the lower edge of said baffie member is connected to said conical head section and the upper edge thereof 1s spaced inwardly from. said cylindrical wall.;

9. In combination with a coking vessel-having a relatively thin metal vertical cylindrical wall and supported by a metal skirt member welded in a relaitvely long narrow section to said wall around the circumference thereof, means for reducing thermal deformation of the welded juncture between said skirt member and said wall which comprises an open-ended cylindrical battle member ra- I dially centered within said vessel and connected to and proximately spaced from the interior surface thereof and opposite said welded juncture, so as to form a coke de'-' positing and retention zone, said bafile member being substantially coextensive with the girth of the welded juncture and extending above and below the juncture and having a Width of 6 to 60 times that of the'metal wall thickness, and an insulating layer of coke in said coke retention zone.

. '10. The apparatus of claim 9 further characterized in the provision of means connecting the upper edge of said v bafile member to said cylindrical wall. I

References Cited in the file of this patent UNITED STATES PATENTS 7 Fields June 28,1932

2,838,381 Daniels JunelO, 1953 I FOREIGN PATENTS 425,290 7 Great Britain F Dec. 18, 1933 

1. IN COMBINATION WITH A PRESSURE VESSEL UTILIZED AS A COKING CHAMBER AND HAVING AN EXTERIORLY EXTENDED VESSEL-SUPPORTING METAL MEMBER WELDED IN A RELATIVELY LONG NARROW SECTION TO A RELATIVELY THIN METAL WALL OF SAID VESSEL, MEANS FOR REDUCING THERMAL DEFORMATION OF THE WELDED JUNCTURE BETWEEN SAID WALL AND SAID SUPPORTING MEMBER WHICH COMPRISES AN INTERNALLY DISPOSED BAFFLE MEMBER CONNECTED TO AND PROXIMATELY SPACED FROM THE INTERIOR SURFACE OF SAID WALL AND OPPOSITE SAID WELDED JUNCTURE, SO AS TO FORM A COKE DEPOSITING AND RETENTION ZONE, SAID BAFFLE MEMBER BEING SUBSTANTIALLY COEXTENSIVE WITH THE LENGTH OF THE WELDED JUNCTURE AND EXTENDING BEYOND EACH LONGITUDINAL EDGE THEREOF AND HAVING A WIDTH OF 6 TO 60 TIMES THAT OF THE METAL WALL THICKNESS, AND AN INSULATING LAYER OF COKE IN SAID COKE RETENTION ZONE. 